The present invention relates to regulating the pilot pressure that is provided as pilot pressure to a manually controlled pilot valve, which when opened in turn provides pilot pressure to operate main control valves to control flow of hydraulic fluid under pressure to various hydraulic components. Specifically the pilot valves control operation of ground drive hydraulic motors for an excavator, or another industrial vehicle, such as a skid steer loader. The manual control can be joysticks, thumb switches, pivoting operator levers or pedals.
Joysticks that control pilot valves are used for operating various hydraulic components at the present time, such as hydraulic motors that are used for ground drives and cylinders that are used for implement functions. Other controls also are used on vehicles for operating pilot valves that would in turn provide a pilot pressure to a pilot operated main valve, to control the vehicle including the speed and direction. In the case of skid steer loaders, a pump for the drive motors is controlled to limit its displacement under high load conditions by reducing the pressure available to stroke a drive motor servo piston or servo controlled pump, or a variable output main valve or control, such as a swash plate. The position of the servo piston or other controlled component in turn determines the flow rate of the pump, which in turn determines the speed of operation of the travel motor. The travel pump presently has a mechanical pressure reducing valve at its input to limit the pilot pressure supplied to the travel controllers when loads on the ground drive are high. The pilot pressure is controlled by a pump speed sensing hydraulic valve, that supplies a pilot pressure that is linearly proportional to the pump speed, which means that as the pump slows down from high loads, the pilot pressure for the travel motor pilot operated valves is reduced. In turn this limits the flow or output of the pilot valve control servo piston or pumps or the controlled valve, so less power is consumed for travel. This system has drawbacks at low speed throttle positions, because the pump speed sensor valve cannot distinguish between idle, when the pump is turned slowly by the drive engine, and high loads when the pump slows down from loading. Further, the prior art system cannot compensate well for changes in oil temperature, since oil viscosity is reduced as temperature increases and flow and pressure developed may be less at the same pump speed when the oil is hot.
In the case of pilot valve joystick controls, such as that on a mini-excavator, the pressure to pilot valves controlled by the joystick presently flows through an on/off solenoid valve, so that when the solenoid valve is on, full pilot pressure is provided to a pilot pressure operated main control valve or servo piston/pump, and when the solenoid valve is off, there is no pilot pressure and the pilot operated main controls are not moved, so travel is stopped.
It is desirable to limit the horsepower requirements of the hydraulic system so that the engine does not stall under any operating condition.